A Multiple Input Multiple Output (MIMO) system is a system that improves data transmission and reception efficiency using multiple transmit antennas and multiple receive antennas. MIMO technology includes a spatial diversity scheme and a spatial multiplexing scheme. The spatial diversity scheme is suitable for data transmission by a User Equipment (UE) that moves at a high speed since transmission reliability or cell coverage can be increased using diversity gain. The spatial diversity scheme can increase data transfer rate without increasing system bandwidth by simultaneously transmitting different data.
In a MIMO system, each transmit antenna has an independent data channel. A receiver estimates a channel with respect to each transmit antenna and receives data transmitted from each transmit antenna. Channel estimation refers to a process of compensating for signal distortion due to fading so as to restore the received signal. Fading refers to a phenomenon in which the intensity of a signal is rapidly changed due to multi-path and time delay in a wireless communication system environment. A reference signal known to both a transmitter and a receiver is necessary for channel estimation. The reference signal may be abbreviated to RS or referred to as a pilot signal depending on the standard.
A downlink reference signal is a pilot signal for coherent demodulation, such as a Physical Downlink Shared Channel (PDSCH), a Physical Control Format Indicator Channel (PCFICH), a Physical Hybrid Indicator Channel (PHICH), and a Physical Downlink Control Channel (PDCCH). The downlink reference signal includes a Common Reference Signal (CRS) shared among all UEs in a cell and a Dedicated Reference Signal (DRS) for a specific UE. The CRS may be referred to as a cell-specific reference signal. The DRS may also be referred to as a UE-specific reference signal.
The DRS is used to provide coherent demodulation of a UE that performs beamforming. CRS-based channel estimation is performed by interpolating and averaging RSs in the time domain and the frequency domain excluding an allocated bandwidth. The UE measures the CRS and provides feedback information such as Channel Quality Information (CQI), a Precoding Matrix Indicator (PMI), and a Rank Indicator (RI) to a base station (or eNB). The base station may perform downlink frequency domain scheduling using the feedback information. A pseudo-random sequence may be used as each of the DRS and the CRS.
A number of considerations should be taken into account when arranging the DRS and the CRS. Such considerations include the amount of radio resources to be allocated to an RS, the exclusive arrangement of the DRS and the CRS, the positions of a synchronization channel (SCH) and a broadcast channel (BCH), the density of the DRS, and the like.
If a large amount of resources are allocated to an RS, data transfer rate may be reduced while it is possible to achieve high channel estimation performance since the density of the RS is high. If a small amount of resources are allocated to an RS, channel estimation performance may be reduced since the density of the RS is decreased although it is possible to achieve a high data transfer rate.
In the case of beamforming transmission, there is a need to use a method of arranging a DRS and a CRS such that the DRS and the CRS do not overlap since both the DRS and the CRS are transmitted. In the case where a specific Orthogonal Frequency Division Multiplexing (OFDM) symbol is allocated for transmission of an SCH and a BCH, it is not possible to transmit a DRS. The UE may fail to restore data if the DRS is not transmitted exclusively with the CRS or is transmitted such that the DRS overlaps the SCH or the BCH.
Accordingly, there is a need to provide a method for efficiently transmitting the DRS.